Stabilization Of Vertically Unstable Pelvic Fractures With External Fixation Through The Iliac Screw Passage: A Biomechanical Study

Objective:To study biomechanical stability of the external fixator through the iliac screw passage which developed by ourselves in the treatment of vertically unstable pelvic fractures and compare biomechanical differences when the pins insert halfway to the PSIS with full inserted.Methods:1 The development of the external fixation through the iliac screw passage(Fig.2)The external fixation we developded are composed of two pieces of 30cm Schanz,two pieces of 38cm Schanz and two bars.The schanz screws are inserted from AIIS to PSIS(the iliac screw passage).The 30cm Schanz screws which have a 7cm thread are inserted into the iliac screw passage around the depth of 7cm.When Schanz screws are fully inserted into the iliac screw passage,we use 38cm Schanz screws with 7cm thread. The two bars were used for connecting the schanz screws on the both sides.The 32cm bar,which has 7cm thread on both ends,was placed at a distance of 10cm from AIIS in the front.A fixation clamp with two female screws on besides sits on each end of the thread and could facilitate compression or distraction.The other bar that is 44cm long and has 7cm thread,share the same constracture on the rest compared the first one and was fixed 20cm away from AIIS in the front.Materials of the strcucture are high-strengthened stainless steel which has good anti-bending ability and is not easy to happen elastic deformation.The pelvic compessor construct place the fulcrum in itself and avoid the open - close conflicts when using a sing-bar to stabilize the pelvis.The adjustment on female screws of the bar can shorten the effective length of a shorter bar and can extend the effective length of a longer bar.AT the same time,the compression across the pubic symphysis and the sacroiliac joint has been achieved and the biomechanical strength in the fixation of the pelvis is also increased.2 The biomechanical studyThree embalmed pelvis were studied.These specimens all were from men,along with their spines intact from the fifth lumbar vertebra to the sacrum.the proximal 1/3 of femora,the intact major pelvic ligaments,and the intact hip joints were obtained.The mean age was 48 years,range 40～60 years.All of these specimens were visually examined and then X-rayed,CT examination to exclude the diseases of pelvic bone such as obvious osteoporosis,tuberculosis,tumor and other pathologic abnormality.The pelvis was cleaned off soft tissues and muscles,retained the integrity of the symphysis pubis and bilateral sacroiliac joints,hip joints,sacrotuberous ligament and sacrospinous ligament,remo-valed intervertebral discs of fifth lumbar vertebra,revealed sclerotin.a platform was made on fifth lumbar vertebra with mixed denture base resin.The platform with a diameter of 2.5 cm steel ball dent was paralleled with the horizontal surface in standing.The study has used double-leg stance model and applied axial loading through the lumbar spine to simulate weight bearing.Two markers was placed at both sides of the sacroiliac joint and pubic symphysis in order to facilitate the displacement measurements.The pelvises were loaded vertically in the CSS-44020 vitodynamics machine and the displacements of Sacroiliac joints and pubic symphysis will be measured.In order to make the models of vertically unstable pelvic fractures,the pelvis was cut off the sacrotuberous ligament and sacrospinous ligament and the posterior sacroiliac ligaments.The normal pelvis was test in the first with the pressure from ON,gradually increasing at a rate of 10N/S.At the pressure of 300N,400N,500N,600N,the displacement of the public symphysis and sacroiliac joint was recorded.then,vertical unstable pelvic fractures modeles were made and the specimens were fixed following by regular external fixation,the external fixation with half-pins through the iliac screw passage,the external fixation with full-pins through the iliac screw passage, the pelvic C-clamp.The specimens were loaded in the same way and the displacement of public symphysis or sacroiliac joint was recorded.During the testing procedure,the specimens were kept moist with saline solution to decrease the organism denaturation owing to the test,so that the experimental results were precise and commeasurable.External fixation should be adjusted before each measurement in order to ensure that the pelvis completely reset.After each test,let the specimen revert to the initial state by taking a 5 minutes rest in order to eliminate the effect because of the creep.To input the five groups data and carry on statistics analysis with spssl3.0(one way ANOVA).Results:With the load pressure gradually increased,the movement of sacroiliac joints and symphysis pubis are gradually increasing.The comparison of different external fixation on stability of sacroiliac joint:The groups with the external fixator through the iliac screw passage(half or fully inserted) and the group with general external fixation were significantly different than that of the normal group,there was significant difference when compared groups under the external fixator through the iliac screw passage(half or fully inserted)with the group under the general external fixation.There was no significant difference between specimens with C-type clamp and specimens with the normal group.The difference was not statistically significant between the pins insert halfway to the PSIS and full inserted in the external fixators through the iliac screw passage.The comparison of different external fixation on stability of pubic symphysis:The group with the C-type clamp,The groups with the external fixator through the iliac screw passage(half or fully inserted) and the normal group were significantly different than the group with general external fixation.There was no statistically significant difference between various external fixation and the normal group exclude the group with general external fixation.Conclusions:The external fixation,which consists of two bar mounted on pins through the iliac screw passage,allows for compression across the pubic symphysis and the sacroiliac joint and could avoid the open - close conflicts when using a sing-bar to stabilize the pelvis,regardless of whether the pins are placed halfway or completely to PSIS.The C-clamp may provide more compressive force to posterior pelvic stabilization but is often more dangerous.The compression construct,a simple adjustment to standard external fixation and previous supraacetabular external fixation,is more suitable for vertical unstable pelvic fractures on account of their better biomechanical advantages and may allow for improved reduction and stabilization of the sacroiliac joint in the acute management of pelvic fractures.However,the pelvic compressor is intended as a temporizing measure in the unstable patient and can not meet the weight-bearing requirements,we do not advocate it as a substitution for definitive posterior internal fixation,excluding certain conditions.